LU-11897 ost: improve memory allocation for ost

LU-11897 ost: improve memory allocation for ost

OST_BUFSIZE is defined as 17 KiB. Lustre uses
OBD_CPT_ALLOC_LARGE() to allocate buffers, which,
in turn, uses kmalloc_node(). kmalloc_node(8192+) falls
back to the traditional buddy allocator kmalloc_large_node().

In the end, 32 KiB is allocated using a 17 KiB allocation
request.

This patch changes OST_BUFSIZE to 32 KiB so we can
effectively use the whole allocated buffer.

Change-Id: I93ce5b26eff4a6a1a17b2a9bfb83161528570197
Signed-off-by: Andrew Perepechko <c17827@cray.com>
Reviewed-by: Alexey Lyashkov <c17817@cray.com>
Reviewed-by: Alexander Zarochentsev <c17826@cray.com>
Tested-by: Alexander Lezhoev <c17454@cray.com>
Cray-bug-id: LUS-6657
Reviewed-on: https://review.whamcloud.com/34127
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Jenkins
Tested-by: Maloo <maloo@whamcloud.com>